In cellular networks for wireless communication, interference may occur in a cell caused by signals transmitted in nearby located cells, which is a well-known problem. In such a network, the available radio bandwidth is limited and in order to provide capacity for communications in the network having multiple cells, resources pertaining to radio bandwidth must be reused in cells at a sufficient mutual distance so as to not disturb communication for one another. In this context, cells that are located near a serving cell are often referred to as “neighbouring cells” or “adjacent cells” and these terms will be used here in the sense that transmissions in neighbouring or adjacent cells may potentially disturb transmissions in the serving cell, and vice versa, thus causing interference. It should be noted that in this context a neighbouring or adjacent cell is not necessarily located right next to the serving cell but may be located one or more cells away, still causing interference.
The following description is relevant for cellular networks using e.g. any of the following radio access technologies: Orthogonal Frequency Division Multiplexing (OFDM), Single Carrier-Frequency Division Multiple Access (SC-FDMA), Orthogonal Frequency Division Multiple Access (OFDMA), Time Division Multiplex (TDM), Frequency Division Multiplex (FDM) and Code Division Multiple Access (CDMA).
A general problem in such cellular networks is that performance in radio communications will be degraded due to interference, e.g. when the same radio bandwidth is used simultaneously in multiple adjacent cells. This problem is typically more common for so-called cell edge terminals, i.e. terminals located close to the cell border and thus also close to neighbouring cells in the vicinity, as opposed to terminals located closer to the cell center and thus not as close to the neighbour cells. In order to address these interference related problems, various so-called Inter-Cell Interference Coordination (ICIC) schemes have been devised where transmissions in adjacent cells are coordinated amongst the cells such that simultaneous transmissions in the same radio bandwidth are avoided or at least restricted. Some examples of ICIC schemes are briefly outlined below.
A so-called High Interference Indicator (HII), referring to uplink resource allocations for cell edge terminals in a first cell, may be sent to the base stations of one or more neighbouring cells. The HII basically indicates that a certain set of uplink radio resources will be allocated to cell edge terminals in the first cell. As cell edge terminal are primarily affected by inter-cell interference, a neighbouring base station receiving the HII can thus avoid allocating radio resources from the same set to its own cell edge terminals.
A so-called Overload Indicator (OI), referring to uplink interference experienced in the first cell, may further be sent to the base stations of one or more neighbouring cells. The OI basically indicates that the current interference level on a certain set of radio resources exceeds a certain threshold in the first cell. In response thereto, a neighbouring base station can thus reduce the interference from the neighbouring cell in the first cell by allocating a different set of resources to its own terminals, or by allocating the interference generating set of resources only to terminals close to the cell center and not to cell edge terminals. The HII can be seen as a proactive ICIC scheme while the OI is a reactive one. Further, the HII and the OI can be exchanged between base stations on the well-known X2 interface, if used such as in Long Term Evolution (LTE) networks.
Further existing ICIC schemes include exchanging a so-called Relatively Narrow band Transmission Power Indicator (RNTPI) referring to restrictions in downlink power, between neighbouring base stations. The RNTPI implies restrictions of transmission power in a certain part of the used radio bandwidth. A base station receiving this indicator may thus allocate radio resources for downlink transmissions within this band and restrict the transmit power accordingly.
The above ICIC schemes rely on information exchanged between base stations, e.g. on the X2 interface. Other ICIC schemes are autonomous in the sense that decisions regarding resource allocation and transmit power are taken internally within the base station without relying on information provided from base stations in neighbouring cells. For example, a scheme called Fractional Frequency Reuse (FFR) can be applied, at least for cell edge terminals, amongst a predetermined set of neighbouring cells such that a certain frequency band is used by the cells in turn, i.e. without overlapping with one another in time.
Other autonomous ICIC schemes include Start Index and Random Start Index which a cell can apply in coordination with a predetermined set of neighbouring cells. In the Start Index scheme, resource allocations within a cell start from a given Physical Resource Block (PRB) index and follow a given direction of a predefined PRB sequence so as to avoid or at least reduce transmission overlaps between the cells. The resource allocations can also be done within opposite PRB sequence directions, referred to as bidirectional Start Index. The Random Start Index scheme is similar to the Start Index, apart from that resource allocations within a cell start from a random PRB index.
The above ICIC schemes entail various restrictions in the usage of radio resources to limit the effects of interference between cells. However, these restrictions of radio resource usage also result in reduced capacity as compared to when all available radio resources can be used for communications in the cell. It is therefore a problem that the above ICIC schemes and others are sometimes employed without much effect on the interference between cells, while still significantly reducing capacity in the cells. Other drawbacks with employing an ICIC scheme to no avail is that it requires some amount of processing and communication over the link between base stations, e.g. the X2 interface, for achieving the coordination between cells, and it may also delay the scheduling and transmission of data resulting in deteriorated radio communication.
On the other hand, it could be quite helpful to use a suitable ICIC scheme when really effective to combat interference such that the use of radio bandwidth among neighbouring cells can be improved to achieve the greatest possible capacity in the network. A problem is therefore to know whether the net effect of using an ICIC scheme is potentially helpful or not for reducing inter-cell interference.